Microlithography has become a widely utilized method for accurately reproducing predetermined images on selected substrates. In this process as taught by the prior art a photoresist is applied to the surface of a suitable substrate such as glass, paper, plastic, metal foil or the like. The coated substrate is then selectively exposed to radiation to cause the exposed portions to become either more soluble or less soluble depending on the type of resist employed. The photoresist is then developed in known manner. If a negative resist is employed, that is, a resist which becomes more insoluble as a result of exposure to radiation, and a suitable pigment or other similar material such as a phosphor is blended with the photoresist as applied using the above-described process, sharp half-tone reproduction can be produced. This method is currently used in half tone photolithographic printing and in the manufacture of monochromatic television picture tubes.
When it is desired to microlithographically produce multi-color images or patterns on a substrate, substantial additional problems in the reproduction process are encountered. The prior art processes require multiple applications of photoresists, one for each color pigment or color phosphor which is to be applied. This results in substantially increased cost and also introduces technical problems in aligning or registering each of the required images with the other images. These problems are encountered in all the prior art processes where multi-coatings of colors and multiple exposure to radiation are required, such as in color photolithographic printing. However, for the purpose of describing the invention, particular attention will be directed to the manufacture of multi-color fluorescent screens for cathode ray tubes. It will be appreciated that the same general procedures are readily employed with other products.
The cathode ray tube, or as it is more commonly known, the picture tube of a color television receiver, includes a glass faceplate on which there is formed the viewing screen. The viewing screen consists of groups of symmetrically placed lines or dots on phosphor powders which, when excited, emit different colors. A one line pattern or one dot pattern on the screen includes individual separate lines or dots of different color-emitting phosphors. Each line pattern or dot pattern generally consists of three separate lines or separate dots, respectively, of red, green, and blue color-emitting phosphors which are the primary color phosphors used to produce colored images on the viewing screen of cathode ray tubes. A black material can also be used to provide a black matrix about the color phosphor areas. Typically, the number of separate areas which can be excited during a full scan of a cathode ray tube having a diagonal width of about 50 centimeters will be about 500,00 to 1,000,000 or even higher. As the number of separate discrete patterns of different colored phosphors which can be excited on the screen increase, the picture generally becomes brighter and the color reproduction of the image becomes more accurate.
When a cathode ray tube is fully assembled for use, for example in a color television receiver, a thin perforated metal plate called a shadow mask is placed a short distance from the screen. The shadow mask has a large number of critically spaced apertures which are in alignment with the phosphor areas on the screen. When the screen is scanned with an aimed electron beam, only selected colored phosphor areas on the screen are excited. The scanning of the screen of the cathode ray tube with an appropriate number of signal carrying electron beams causes certain selected phosphor areas to be excited and emit colors and this results in the production of a colored image on the screen of the cathode ray tube.
Various problems are encountered in the manufacture of multi-color cathode ray tubes. The discrete line or dot patterns, which are formed on conventional cathode ray tubes, are already numerous and the trend is to attempt to even more closely pack a larger number of patterns of the different color-emitting phosphors on the screen. To obtain the desired improvement in quality the phosphors must be applied in close proximity to each other and must also be present in the discrete areas in exact registration with each other and with adjacent repeats of the pattern. The different color-emitting phosphors cannot overlap each other, that is contaminate each other, as this destroys the color fidelity of the picture produced on the screen.
The technical problems which are encountered in the manufacture of cathode ray tubes are numerous and these are further complicated by commercial considerations. It is essential that any commercial process for the formation of the screens must be relatively low in cost per unit so as to make large scale production feasible. The required low-cost, mass-production methods must also result in a high quality and extreme uniformity without requiring extremely rigid process controls or highly critical and sensitive production procedures.
Various methods have been suggested in the prior art for applying the phosphors to the screens of the cathode ray tubes. One such method is disclosed in Bowerman, U.S. Pat. No. 2,854,348, wherein the surface screen is printed with a tacky material and then the surface is dusted with a suitable powdered phosphor. The printing process and the dusting process are repeated for each application of a different phosphor. The patterns which are printed on the screen of the tube must be accurately aligned with each other to prevent overlap of different phosphors and also to provide the proper registration. The printing process has proven to be inherently unsatisfactory because of the physical limitations of printing processes to produce micro-fine separations and accurate spacing of the different colored phosphor areas.
Various methods are used in accordance with the prior art which employ negative photobinders and selective radiation exposure. A negative photobinder is a material which, on exposure to radiation such as ultraviolet light, becomes relatively insoluble and nonadhesive in comparison to unexposed portions. Some conventional negative photobinders which are used in the manufacture of cathode ray tubes are chromate sensitized casein and chromate sensitized vinyl alcohol polymers. The negative photobinder is applied to the faceplate of the cathode ray tube and then the entire area of the faceplate is exposed with the exception of those areas where it is desired to apply the phosphor. The exposed portions become nonadhesive and relatively insoluble. Then, by applying a suitable solvent, the unexposed areas can be made relatively tacky. The relatively tacky areas are then dusted with a phosphor which selectively sticks to the unexposed areas. When applying a number of different colored phosphors utilizing the above prior art technique, it is necessary to apply a new coating of the photobinder for each color phosphor which is applied.
An alternate method which has been suggested in the prior art is to apply a mixture of a negative photobinder such as those identified above and the particular type of phosphor desired to be applied to the surface of the screen either as a slurry or as a dry powdered mixture in a manner to adhere it uniformly to the screen. Using this technique, the photobinder is exposed through a shadow mask in those areas wherein it is desired to adhere the phosphors to the screen. The exposure to radiation causes the exposed areas of the photobinder to become relatively insoluble. Thereafter, the entire surface of the photobinder is developed to remove the unexposed materials. The coating procedure, the exposure and the development are repeated for the application of each type of phosphor. Such a process is disclosed by Veirs, U.S. Pat. No. 3,544,350. There are various modifications of the above processes, such as those disclosed in Lange et al., U.S. Pat. No. 3,597,258, and Saulnier, U.S. Pat. No. 3,981,729.
The prior art methods have the common problem of requiring multiple application of binder. In addition, certain of the processes require development steps between each application of phosphor as well as other similar intermediate steps such as drying and conditioning.
The production costs are increased because of the multiple steps. In addition, the chances for error in placement and cross-contamination of the phosphors is likewise significantly increased because of the multiple steps.
It would be highly advantageous if a microlithographic method could be provided for the manufacture of multi-colored products such as colored cathode ray tubes, multi-color photolithographic prints and the like which would involve fewer process steps and which provided improved accuracy of reproduction.